1. Field of the Invention
The present invention relates to a fuel cell stack, particularly to facilitating its startup under freezing conditions.
2. Description of Related Art
Conventionally, a fuel cell generates heat of reaction and water in power generation. In a normal power generation state, a cooling device is provided therein to prevent the heat of reaction from raising the temperature of the fuel cell more than necessary. However, it is known that when starting the power generation under freezing conditions, the fuel cell is required to have a constant temperature, otherwise power generation efficiency drops off.
This is because, under the freezing conditions, water generated by the power-generating reaction freezes on a surface of an electrode so as to prevent oxygen supply to the electrode. Such freezing of the generated water is caused by absorption of the heat of reaction produced by the power generation into a separator of the fuel cell. Therefore, it has been proposed that a heat generator such as a heater be separately provided to start the power generation, as described in Japanese Patent Laid-Open No. Hei 7-94202, and that the power output is gradually increased from a low level at the start of power generation to prevent the generated water from freezing, as described in Japanese translation of PCT publication No. 2000-512068.
However, providing a separate heat generator consumes extra electric power, and makes the device large-sized. In addition, there is also the problem that increasing the power generation gradually requires a long time to reach a normal state.